Mike
I have the same car (98 Grand Marquis de Sade) but I got plenty of heat.
If you want, stop by my house and you can have my Merc. It needs tires, exhaust system, cat converter, ac compressor, horn does not work and wipers are a little screwy, but it has tons of heat. Valid sticker ??? Fagetabodit.
Only 130K miles so its just broken in (just a little bit of rust). I just doubled the sale price cuz I filled the tank with gas.
A cheaper alternative is to put a big aluminum turkey pan (the cooking type, not the Cobra air cleaner type) in the passenger seat footwell, add some newspaper and small twigs ... a single match and viola ... heat. It works, especially in 1968 Ramblers ,,, ask me how I know.
